Regulation of 3-hydroxy-3-methylglutaryl-CoA reductase by endogenous sterol synthesis in cultured intestinal mucosa

The effects of atorvastatin on gluten-induced intestinal T cell responses in coeliac disease

Various experimental models suggest that the cholesterol-lowering drugs statins may also modulate immune responses. Cellular level studies on human disorders are needed, however, to provide a rational basis for clinical testing of statins as immune therapy. Coeliac disease, a chronic small intestinal inflammation driven by HLA-DQ2 restricted mucosal T cells that are specific for ingested wheat gluten peptides, is in many ways ideal for this purpose. In addition, there is a need for alternative treatment to the gluten-free diet in this disorder. Here we have assessed the effects of atorvastatin on gluten-reactive T cells, dendritic cells and the coeliac mucosa by in vitro culture of biopsies. Atorvastatin inhibited gluten-induced proliferation and specific cytokine production of human intestinal gluten-reactive T cell clones and lines. Dendritic cells exposed to atorvastatin displayed a reduced expression of the costimulatory molecule CD83 upon maturation with lipopolysaccharide. Incubation of intestinal biopsy specimens with atorvastatin in vitro, however, did not influence gluten-induced cytokine release. In conclusion, atorvastatin has specific effects on isolated gluten-reactive T cells and dendritic cells, but does not shut down the gluten-induced production of proinflammatory cytokines in intestinal biopsies.

Processing and characterization of the low density lipoprotein receptor in the human colonic carcinoma cell subclone HT29-18: a potential pathway for delivering therapeutic drugs and genes

Low density lipoprotein (LDL) processing has been investigated in the subcloned human colonic carcinoma cell line HT29-18. LDL binding at 4 degrees C was a saturable process in relation to time and LDL concentration. The Kd for LDL binding was 11 micrograms/ml. ApoE-free HDL3 or acetylated LDL did not significantly compete with 125I-LDL binding, up to 500 micrograms/ml. 125I-LDL binding was decreased by 70% in HT29-18 cells preincubated for 24 hours in culture medium containing 100 micrograms/ml unlabelled LDL. Ligand blotting studies performed on HT29-18 homogenates using colloidal gold labelled LDL indicated the presence of one autoradiographic band corresponding to an apparent molecular weight of 130 kDa, which is consistent with the previously reported molecular weight of the LDL receptor in human fibroblasts. At 37 degrees C, 125I-LDL was actively internalized by HT29-18 cells and lysosomal degradation occurred as demonstrated by the inhibitory effect of chloroquine. LDL uptake and degradation by HT29-18 cells also resulted in a marked decrease in endogenous sterol synthesis. These data demonstrate that the HT29-18 human cancerous intestinal cells are able to specifically bind and internalize LDL, and that LDL processing results in down-regulation of sterol biosynthesis. Thus, intestinal epithelial cells possess specific LDL receptors that can be exploited to accomplish drug delivery and gene transfer via the receptor-mediated endocytosis pathway.

Mevinolin, a competitive inhibitor of hydroxymethylglutaryl coenzyme A reductase, suppresses enterocyte esterification of exogenous but not endogenous cholesterol

Mevinolin (lovastatin), a competitive inhibitor of hydroxymethylglutaryl-coenzyme A reductase, directly inhibited acyl-CoA cholesteryl acyltransferase in rabbit intestinal microsomes at a dose of 20 micrograms/ml or more. Lineweaver-Burk analysis showed a competitive type of inhibition with respect to oleoyl-CoA. In cultured intestinal Caco-2 cells, mevinolin reduced [14C]oleate incorporation into cholesteryl-esters by 86% of controls at doses as low as 0.1 micrograms/ml. However, in cells whose activity of acyl-CoA cholesteryl acyltransferase was stimulated 7-fold by 10 mM mevalonolactone, a significant inhibitory effect on cholesteryl-ester formation could not be detected, even at 40 micrograms/ml of mevinolin. In contrast, cells supplied with liposomal cholesterol or cholesterol derived from low-density lipoproteins showed a marked reduction of cholesteryl-ester formation in the presence of 10 or 0.1 micrograms/ml of mevinolin, respectively. It is concluded that the observed suppressive effects of mevinolin on cholesterol esterification in cultured Caco-2 cells are indirect and possibly caused by changes in the acyl-CoA cholesteryl acyltransferase substrate pool or intracellular cholesterol transport.

Fate of exogenous and newly synthesized cholesterol in intestinal cell lines

1. The current study was undertaken to test the existence of functionally distinct intracellular pools of cholesterol depending on the origin: neosynthesis or exogenous. 2. This was performed on two subpopulations, either differentiated or undifferentiated, of the HT29 cell line. 3. A parallel study was also carried out on Caco-2 cells. 4. First we checked the ability of differentiated HT29 cells to secrete lipids into the medium and found that lipid production was efficient but less so than in Caco-2 cells. 5. In contrast, undifferentiated HT29 cells were unable to secrete lipids into the medium. 6. Then we studied the fate of [14C]cholesterol incorporated into micellar preparations and of [14C]mevalonate in the different models. 7. The data obtained with labelled exogenous cholesterol show that it enters the membrane cholesterol pool as well as, for the differentiated models, the cholesteryl ester pool. 8. Similarly, labelled newly synthesized cholesterol could be used for membrane formation as well as for incorporation into cholesteryl esters. 9. Thus, in HT29 subpopulations as well as in Caco-2 cells, the results suggest the existence of a common pool of cholesterol whatever its origin.

Effect of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibition on human gut mucosa

Mevalonic acid is an important biochemical intermediate in cholesterol synthesis and other processes involved in cell replication. 3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase is the enzyme which catalyzes mevalonic acid synthesis. To determine whether a potent competitive inhibitor of this enzyme, the drug simvastatin, may have an adverse effect on enterocyte cell replication and cholesterol metabolism, small intestinal biopsies from nine hypercholesterolemic subjects were obtained before and during treatment with simvastatin as a lipid-lowering agent. Histologic review of biopsies in a blinded manner detected no change in ratio of villous length to crypt length or in mitotic index which might indicate altered cell replication. Similarly, no significant change in measured activity of HMG-CoA reductase activity was observed. In spite of the high exposure of jejunal mucosal cells to this potent competitive inhibitor of a key enzyme, no adverse effect on growth could be detected.

In vitro effects of arachidonic acid on the prostaglandin synthesizing system in gastric mucosa

Self-destruction of the prostaglandin cyclooxygenase has been suggested to be an important factor in the regulation of endogenous prostaglandin synthesis. The present study was done in order to define the role of this substrate-induced inactivation in the regulation of prostaglandin synthesis in gastric mucosa. In tissue homogenate, the prostaglandin synthesizing capacity is rapidly inactivated at 37 degrees C, even in the absence of exogenous arachidonic acid. It was shown that this inactivation can be prevented both by EDTA as a chelator of calcium-ions and by tetracaine, a specific inhibitor of the phospholipase A2. Additional exogenous arachidonic acid again inactivated prostaglandin synthesis in a dose dependent manner. In contrast, the prostaglandin synthesizing capacity in organ cultured mucosal biopsies is well preserved, although the release of endogenous substrate was activated by extracellular calcium and Ca-ionophore A23187. Furthermore, even at high concentrations of exogenous arachidonic acid present in the culture medium, the synthesizing capacity in intact biopsies was only slightly and reversibly reduced. These large differences between intact biopsies and cell free tissue preparations point to very efficient mechanisms controlling the substrate availability for the cyclooxygenase system both from endogenous and exogenous sources in intact gastric mucosa.

Physiological and pharmacological regulation of small intestinal cholesterol synthesis

1. The small intestine is an important site of cholesterol synthesis in the body and at least in experimental animals, it also contributes to the circulating plasma pool of cholesterol. 2. Studies on synthesis regulation have been partly contradictory but it is now concluded that the cellular cholesterol balance is the basic regulatory factor of intestinal cholesterol synthesis. However, the balance is affected differently in various specialized cells and parts of the small intestine. 3. Most data on synthesis regulation are derived from experimental animals but the few human studies suggest that similar regulatory factors function in man, too.

The role of enterocyte cholesterol metabolism in intestinal cell growth and differentiation

Cholesterol is an essential constituent of all mammalian cell membranes, and its availability is therefore a prerequisite for cellular growth and other functions. To define further the role of cholesterol metabolism in the intestine both in vitro and in vivo, studies were performed. Several lines of evidence based on these studies suggest that the main purpose of local cholesterol synthesis in the gut is the support of rapid enterocyte proliferation: 1) growth was inhibited during pharmacologic suppression of cholesterol synthesis in intestinal organ or cell culture; 2) the endocrine regulation of intestinal growth was in most but not all instances accompanied by appropriate changes in cholesterol synthesis; 3) most of cholesterol synthesis and lipoprotein uptake was localized predominantly in the crypt and lower villus region; and 4) very little of the sterol synthesized by the intestinal mucosa was exported into lymph but seems rather to be incorporated into cell membranes.

Regulation of cholesterol synthesis and binding of lipoproteins in cultured rat intestinal epithelial cells

The regulation of cholesterol synthesis has been studied using a rat epithelial intestinal cell line (IRD 98) as a cellular model. As observed in other cell types, mevinolin increases the levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.34) and concomitantly reduces the incorporation of [14C]acetate into cholesterol. Free cholesterol is able to suppress reductase activity. In contrast, when cells are shifted from standard culture medium to lipoprotein-deficient medium, an increase in hydroxymethylglutaryl-CoA reductase specific activity (2-5-fold) is observed. The possible regulatory roles of the different classes of human lipoproteins were thus compared. The effects of a long-term exposure to LDL and HDL vary according to cell density. In actively growing cells, VLDL and LDL cause a decrease in the level of hydroxymethylglutaryl-CoA reductase, whereas HDL do not have a significant effect. In contrast, in subconfluent preresting cells, HDL provoke large decreases in hydroxymethylglutaryl-CoA reductase activity as compared to VLDL and LDL. While LDL binding is constant, the maximal binding capacity of HDL in subconfluent cells is seven times that of actively growing cells. Altogether, these results suggest an important role for HDL in the regulation of intestinal cholesterol synthesis.

Cholesterol synthesis and esterification in cultured intestinal mucosa evidence for compartmentation

The current studies were undertaken to define the optimal conditions for measuring the absolute rates of cholesterol synthesis in cultured rabbit intestine and to assess whether the rate of sterol synthesis affects the esterification of locally formed or absorbed cholesterol. Using both [3H]water or [14C]octanoate (3 mM) as a precursor, sterol formation was linear during the 24 h culture, resulting in comparable estimates of the rate of synthesis equivalent to 129.5 and 118.7 nmol acetyl CoA incorporated per g per h, respectively. The presence of liposomal cholesterol or the hydroxymethylglutaryl-CoA reductase inhibitor mevinolin suppressed the rates of cholesterol synthesis by 24 and 92% of controls, respectively. Only 12% of total newly synthesized cholesterol was recovered in the medium and more than 97% was in the unesterified form, in both medium and biopsy. Even when the rate of sterol synthesis was stimulated over 90-fold by increasing concentrations of [14C]mevalonolactone, less than 8% of the label in total cholesterol was found in the sterol nucleus of the esterified cholesterol. Rather, the majority of the cholesterol ester-bound radioactivity was incorporated into the fatty acid moiety. On the other hand, there was only a limited decrease in the esterification of absorbed [3H]cholesterol both when the rate of sterol synthesis was increased with 10 mM mevalonolactone and when it was inhibited with mevinolin. The data suggest that locally synthesized and absorbed cholesterol is organized in distinct functional pools with different degrees of esterification in the mucosal epithelial cell.

Bile acid induced interconversion of 3-hydroxy-3-methylglutaryl Coenzyme A reductase in cultured intestine

The effect of bile acids and bile acid/cholesterol micelles on 3-hydroxy-3-methylglutaryl coenzyme A reductase, the key enzyme of cholesterol synthesis, was investigated in cultured intestine. Glycocholic and glycodeoxycholic acid both suppressed total (fully activated) reductase activity after 3 h culture. The portion of expressed reductase, determined in the presence of NaF, was unaffected at 3 h, but decreased after 24 h of bile acid treatment. In contrast, total enzyme activity was stimulated up to 2.5-fold at 24 h; this bile acid effect was blocked by additional cholesterol. These results suggest that bile acids modulate both total reductase activity and the activation state of the enzyme in cultured intestine.